The first step taken by a radio communication device such as a mobile radio unit when commencing communications in a Code Division Multiple Access (CDMA) system is to identify the transmitting base stations or cells in terms of their primary scrambling code and the corresponding frame timing. Prior to commencing communications with a base station, the mobile unit has to synchronize itself with the timing reference of a base station. This process is commonly referred to as cell search. Once the primary scrambling code and frame timing of the target cell has been identified, the mobile unit sets up signaling and user (voice or data) channels to communicate with the base station. In third Generation Partnership Project Frequency Division Duplex (3GPP FDD) Direct Sequence (DS) systems, often referred to as Wideband Code Division Multiple Access (W-CDMA), the cell search is typically carried out in three main stages as follows:
Stage 1). Slot synchronization: During this first stage of the cell search procedure, the mobile unit uses the Synchronization Channel's (SCH) Primary Synchronization Code (PSC) to acquire slot (e.g., slots occur in 666 μs time intervals) synchronization to a cell as shown in step 102 of FIG. 1. The PSC is a 256-chip length synchronization to a cell as shown in step 102 of FIG. 1. The PSC is a 256-chip length code transmitted every slot and all cells transmit the same PSC. This process involves matched filtering with the PSC. The slot timing of the cell can be obtained by detecting peaks in the matched filter output, relative to the receiver time reference. A diagram highlighting the structure of the SCH channel is shown in FIG. 2, where: cp is the Primary Synchronization Code, Csi,k is one of 16 possible Secondary Synchronization Codes (Csi,1, Csi,2, . . . , Csi,15) encode cell specific long scrambling code group “i”, and “a” is the modulation on the primary and secondary synchronization codes used to indicate STTD encoding on the PCCPCH.
Stage 2). Frame synchronization and code-group identification: During the second step of the cell search procedure as shown in step 104 of FIG. 1, the mobile unit uses the SCH's secondary code (SSC) to find frame synchronization and identify the code group of the cell found in the first step. All scrambling codes used in 3GPP systems are divided into groups of eight, and the code group is encoded into a sequence of SSCs. Correlating the received signal with all possible SSC sequences, and identifying the maximum correlation value accomplishes this. Since the cyclic shifts of the sequences are unique, the code group as well as the frame synchronization is determined.
Stage 3). Scrambling code identification: During the third and final step of the cell search procedure as shown in step 106 of FIG. 1, the mobile unit determines the exact primary scrambling code used by the cell it has found. The primary scrambling code is typically identified through symbol-by-symbol correlation over the common pilot channel (CpiCH) with all eight codes within the code group identified in the second step.
If the mobile unit has received a priority list with information about the set of scrambling codes to search for, steps 104 and 106 above can be simplified. This particular situation can arise in most cases other than when the unit is performing the first “power-on” acquisition sequence. However, if the scrambling code is known, but the cell using that code is not the strongest cell, it may take several iterations of steps 104 and 106 on a number of peaks identified in step 102, to find the target cell. This is because, after step 102, there is no way of distinguishing between peaks due to multi-paths from an already identified cell and paths from the new target cell. So, after step 102, there may be in the order of 100 or more hypotheses to search through using steps 104 and 106, assuming a UE can, in the worst case, receive paths from up to twenty base stations, and an average of five paths from each.
A totally different approach that does not use steps above can be taken to establish frame timing, in the scenario where the scrambling code is known. If the scrambling code or a set of possible scrambling codes is known, it is possible to search through the whole uncertainty window with the known scrambling code(s) and identify the correct scrambling code and the corresponding frame timing as shown in FIG. 3. This is similar to the initial acquisition step in an IS-95 system, and requires searching through 38,400 (i.e., number of chips in a frame) chip offsets in order to locate the frame timing.
Both of the above approaches, will result in cell acquisition times of several 100 milliseconds, as well as require a large amount of processing resources, in spite of the fact that the set of possible scrambling codes are known. A need thus exist in the art for a cell search scheme that can help reduce the time and computational resources required to execute a cell search routine.